Bonding modes and gratuitous arp

KJoubert · 08-14-2017, 01:39 PM

Hello. I have a question about the Linux bonding modes and gratuitous arp.

Based on the bonding documentation that I can find (it seems unchanged since 2011, https://www.kernel.org/doc/Documenta...ng/bonding.txt)...

when using bond mode 1, a gratuitous arp is sent in the event of a failover (bond leg or cluster node). I see no reference to this when using bond mode 6, which seems to use ARP negotiation instead and responds with ARP replies to correct ARP entries on a requested basis, rather than the broadcast that occurs when using bond mode 1.

I have an issue where servers are using bond mode 6, and the network engineers are complaining about a lot of log traffic after failover, indicating MAC address changes. It doesn't interrupt the client connection to my knowledge... but it does generate log traffic and alerts for the engineers.

Their request is... "Hey... we were expecting a gratuitous ARP and didn't get one? Why not?"

Based on the documentation and my own very limited networking knowledge, it doesn't look like a gratuitous ARP is sent unless the servers are configured for bond mode 1 (Active/Backup). Ours are configured for bond mode 6 (ALB).

Clear as mud?

Any input appreciated. Thanks.

MensaWater · 08-15-2017, 10:50 AM

Quote:

Mode 6 (balance-alb)
This is Adaptive load balancing mode. This includes balance-tlb + receive load balancing (rlb) for IPV4 traffic. The receive load balancing is achieved by ARP negotiation. The bonding driver intercepts the ARP Replies sent by the server on their way out and overwrites the src hw address with the unique hw address of one of the slaves in the bond such that different clients use different hw addresses for the server.

There is no gratuitous arp sent because the MAC (hw) address of of the bond[#] device is ALWAYS that of the one of the underlying slave interfaces in the bond it obtained on start.

That is to say the reason one would send a gratuitous arp is to notify neighbors (including switches) that the IP is now on a different MAC (hw) address but in mode 6 it is never on a "different" address than the one it started with. (Except after network restart such as that done on reboot but even then only if it can't activate the slave interface for some reason so picks the address from a different interface.)

KJoubert · 08-15-2017, 12:20 PM

Quote:

Originally Posted by MensaWater

There is no gratuitous arp sent because the MAC (hw) address of of the bond[#] device is ALWAYS that of the one of the underlying slave interfaces in the bond it obtained on start.

That is to say the reason one would send a gratuitous arp is to notify neighbors (including switches) that the IP is now on a different MAC (hw) address but in mode 6 it is never on a "different" address than the one it started with. (Except after network restart such as that done on reboot but even then only if it can't activate the slave interface for some reason so picks the address from a different interface.)

Thank you.

-K